1. Field of the Invention
Relates to processes and apparatus for producing asphaltconcrete and more particularly to apparatus and methods for reducing particulate emission from a dryer-drum mixer in the production of asphalt paving compositions.
2. Description of the Prior Art
Bituminous pavements of the plant mix type have traditionally been produced for the most part by drying aggregate in a drum dryer, feeding the dried aggregate into a screening unit capable of separating the dried aggregate into fractions held in a series of storage bins, feeding the fractions in proportioned amounts from the bin units, and mixing the proportioned amounts with a predetermined amount of bituminous composition in a mixing unit of the pug mill type. The drum dryers used to dry the aggregate are generally heated by a forced air burner. Particulate emissions from dryers of this type have posed a problem due to increasingly stringent air pollution regulations. Dust collectors of the "baghous" or "scrubber" types, costing tens of thousands of dollars per plant, are presently the means used to control particulate emissions from drum dryers used in drying aggregate.
Asphalt paving compositions have also been made in a continuous drum type mixing plant, by feeding aggregate into the mixer, contacting it in the mixer with a bituminous composition in the form of an emulsion, and heating the mixture, during mixture of the aggregate and bituminous composition, with a heated gas stream flowing counter-current to the flow of the aggregate-bituminous composition flow.
More recently, Shearer in his U.S. Pat. No. 3,832,201 disclosed a process and apparatus for making asphalt paving compositions wherein cold, wet aggregate is contacted with a liquefied asphalt composition just prior to or concurrently with introduction of the aggregate into a relatively high velocity, heated gas stream flowing parallel to the direction of the aggregateasphalt mixture through a mixing and drying zone. In the heated atmosphere, the mixture of aggregate and asphalt is mixed while being heated to coat the aggregate particles with asphalt and the coated particles are cascaded through the heated atmosphere to remove moisture therefrom without substantial aging or hardening of the asphalt.
While the basic Shearer process for asphalt production was considered to be pollution free in regard to particulate emission at that time, more stringent environmental protection standards imposed recently in many areas often require greater particulate emission control than is possible with the aggregate supplied locally using the basic Shearer process as originally disclosed.
Therefore, it is an object of the present invention to provide an improved dryer-mixture asphalt process and apparatus which will substantially reduce particulate emissions.
It is still another object of the present invention to provide an improved dryer-mixer asphalt process and apparatus with minimal particulate emissions providing improved thermal efficiency.